The invention relates generally to computed tomography, and more particularly to methods for energy sensitive computed tomography systems that use checkerboard filtering.
Typically, energy-sensitive computed tomography systems employ one of two techniques: acquiring projection data using dual-energy principles, which modulate the spectrum from the X-ray tube by selecting the operating voltage of the X-ray tube or by spectral filtering techniques, or utilizing detector technology to provide energy-sensitive measurements. In one example of the former technique, data is acquired from an object using two operating voltages of an X-ray source to obtain two sets of measured intensity data using different X-ray spectra, which are representative of the X-ray flux that impinges on a detector element during a given exposure time. In a latter technique, energy sensitive detectors such as, but not limited to, photon counting detectors and dual-layered detectors are used. In general, at least one data set is then processed to represent line integrals of the linear attenuation coefficients of the object along paths of X-ray radiation from the source to the individual detector elements. The measured data that are processed are typically called projections. By using reconstruction techniques, cross-sectional images of the scanned object are formulated from the projections. Utilizing both sets of projection data acquired using different X-ray spectra, line integrals of the density distribution within the field of view of the imaging system of two chosen basis materials can be generated. By using reconstruction techniques, cross-sectional images of the density distributions for both basis materials can be formulated or the effective atomic number distribution within the field of view of the imaging system computed.
X-ray beam attenuation caused by a given length of a material, such as, but not limited to, bone or soft tissue, may be represented by an attenuation coefficient for that material. The attenuation coefficient models separate physical events that occur when the X-ray beam passes through a given length of the material. A first event, known as Compton scatter, denotes the tendency of an X-ray photon, passing through the length of the material, to be scattered or diverted from an original beam path, with a resultant change in energy. A second event, know as photoelectric absorption, denotes the tendency of an X-ray photon, passing through the length of the material, to be absorbed by the material. There are other physical processes that may occur, but given the X-ray energies present in the spectra, their effect is insignificant relative to the two events listed above.
Different materials differ in the scatter and absorption properties, resulting in different attenuation coefficients. In particular, the probability of Compton scattering depends in part on the electron density of the imaged particle and probability of photoelectric absorption depends in part on atomic number of the imaged material, i.e., the greater the atomic number, the greater the likelihood of absorption. Furthermore, both Compton scattering and photoelectric absorption depend in part on the energy of the X-ray beam. As a result, materials can be distinguished from one another based upon relative importance of photoelectric absorption and Compton scattering effects in X-ray attenuation by the material. A density distribution and an effective atomic number distribution may be obtained using the two sets of projection data. However, the technique has limitations due to a slow acquisition mechanism since projection data sets corresponding to two separate energy spectra from the X-ray tube must be measured. This limitation can be overcome by rapidly alternating the operating voltage of the X-ray tube at alternating view angle positions of the rotating gantry; however, this technique requires special circuitry in the power supply to enable sufficient switching speeds.
Using dual-energy techniques, a density distribution and an effective atomic number distribution may be obtained using the two sets of projection data. However, the technique has limitations due to a slow acquisition mechanism since projection data sets corresponding to two separate energy spectra from the X-ray tube must be measured. This limitation can be overcome by rapidly alternating the operating voltage of the X-ray tube at alternating view angle positions of the rotating gantry; however, this technique requires special circuitry in the power supply to enable sufficient switching speeds.
Projection data acquisition for energy sensitive computed tomography (CT) may be implemented using various techniques that typically involve modification of X-ray generation or X-ray detection. For example, Rotate-Rotate dual-energy acquisitions (e.g., X-ray tube voltage modification on subsequent rotations of the CT gantry), fast-kVp switching of the X-ray tube voltage (i.e., X-ray tube voltage modulation at a periodicity driven by the particular imaging application), dual-layer detectors (i.e., variable X-ray detection) and energy sensitive X-ray detection (e.g., energy sensitive, photon-counting detectors) are such techniques.
Rotate-Rotate dual energy acquisitions require scanning of a volume using two operating voltages of the X-ray tube on subsequent gantry rotations. As a result, axial scanning protocols are required (obviating helical scanning techniques) to ensure comparable X-ray path integrations of the linear attenuations coefficients for both spectra. Since two sequential scans are required, overall scan times are increased with this technique.
“Fast-kVp switching” utilizes rapid switching between two operating voltages of the X-ray tube during scanning, which requires that special circuits be used to transfer stored charge in the high-voltage cable and transformer to enable rapid switching of the X-ray tube voltage.
Some dual-energy detectors use multiple detector layers; the top layer measuring a low-energy spectrum and the bottom layer measuring a high-energy spectrum. As a result, detector cost is effectively doubled. Dual-layered detectors are not cost effective since two separate detectors are needed to generate the requisite projection data.
Energy sensitive detectors detect individual X-ray photons and characterize their energy so that the spectral distribution of the measured spectrum is estimated. The detector cannot accurately measure the spectrum from regions of an object with minimal attenuation as signal pile-up occurs, due to limitations in counting rates provided with current technology.
While all these technologies have identifiable limitations, some provide better fidelity in spectral sensitivity than others. For example, dual-layer detectors provide moderate fidelity in spectral sensitivity; dual-energy acquisition and X-ray tube voltage switching provide good fidelity; while energy-sensitive detectors typically provide the best fidelity in spectral sensitivity. A good metric to evaluate energy sensitive techniques is the separation in mean energy of energy sensitive measurements. As the difference in mean energy increases, the fidelity in spectral sensitivity increases.
Some energy sensitive techniques reduce throughput (dual energy scanners, photon counting detectors) or require more costly hardware (fast kVp switching, dual-layer detectors). Therefore, it is desirable to employ techniques that maintain throughput, allow generation of high-resolution CT imagery that use all of the acquired projection data (i.e., both high and full-spectrum data), and provide the requisite fidelity in spectral sensitivity.